Signal detecting device and optical fiber characteristics measuring device

ABSTRACT

A signal detecting device includes a multiplier that multiplies a measurement signal by a reference signal, a filter that filters a multiplication result from the multiplier, a first storage that stores an internal state of the filter; and a second storage that stores a filtering result from the filter. The filter filters the multiplication result using the internal state stored in the first storage. The first storage switches an area in or from which the filter writes or reads the internal state in accordance with an index signal representing a type of amplitude of a time-divisional signal in the measurement signal. The second storage switches an area in which the filtering result is stored in accordance with the index signal.

BACKGROUND

The present invention relates to a signal detecting device and anoptical fiber characteristics measuring device.

DESCRIPTION OF THE RELATED ART

Generally, a lock-in amplifier includes a multiplier that multiplies ameasurement signal by a reference signal and a low pass filter thatextracts a DC component of a signal acquired through multiplicationusing the multiplier and is a signal detecting device that detects aspecific signal included in a measurement signal. Such a lock-inamplifier has a feature of being able to detect a minute signal embeddedin noise with high sensitivity. As types of lock-in amplifier, there arean analog type in which a multiplier and a low pass filter are realizedusing analog circuits and a digital type in which these are realizedusing digital circuits.

In Japanese Unexamined Patent Application Publication No. 2012-159387and Japanese Unexamined Patent Application Publication No. 2022-014178,an optical fiber characteristics measuring device including a lock-inamplifier is disclosed. This optical fiber characteristics measuringdevice is a device that detects a temperature distribution and adistortion distribution in a longitudinal direction of an optical fiberby detecting Brillouin scattering light generated in accordance withlight incident to an optical fiber. Brillouin scattering light generatedinside an optical fiber is extremely weak, and a lock-in amplifier isused for detecting such weak Brillouin scattering light with highsensitivity.

In a conventional lock-in amplifier, as described above, a DC componentis extracted from a signal acquired by multiplying a measurement signalwith a reference signal using a low pass filter. For this reason, forexample, amplitudes of a signal of which frequencies are the same as afrequency of the reference signal, and the amplitude time-divisionallychanges (a time-divisional signal) cannot be separated and detected.Even when a measurement signal including such time-divisional signals isinput to a conventional lock-in amplifier, only one output acquired byperforming low pass filter processing on the time-divisional signals canbe acquired from the conventional lock-in amplifier.

SUMMARY

A signal detecting device may include: a multiplier configured tomultiply a measurement signal by a reference signal; a filter processor(a filter) configured to perform filter processing on a multiplicationresult acquired by the multiplier; a first storage configured to storean internal state of the filter processor; and a second storageconfigured to store a processing result (a filtering result) acquired bythe filter processor. The filter processor may be configured to performthe filter processing using the internal state stored in the firststorage. The first storage may be configured to perform switching of anarea in/from which the internal state is written/read by the filterprocessor in accordance with an index signal representing a type ofamplitude of a time-divisional signal included in the measurementsignal. The second storage may be configured to perform switching of anarea in which the processing result acquired by the filter processor isstored in accordance with the index signal.

Further features and aspects of the present disclosure will becomeapparent from the following detailed description of exemplaryembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a main configuration of a signaldetecting device according to a first example.

FIG. 2A is a diagram illustrating an example of a measurement signalinput to the signal detecting device according to the first example.

FIG. 2B is a diagram illustrating an example of a reference signal inputto the signal detecting device according to the first example.

FIG. 2C is a diagram illustrating an example of an index signal input tothe signal detecting device according to the first example.

FIG. 3A is a diagram illustrating an example of a memory map of theinternal memory according to the first example.

FIG. 3B is a diagram illustrating an example of a memory map of thememory according to the first example.

FIG. 4 is a block diagram illustrating a main configuration of a signaldetecting device according to a second example.

FIG. 5 is a block diagram illustrating a main configuration of anoptical fiber characteristics measuring device according to one or moreembodiments.

DESCRIPTION OF THE EMBODIMENTS

The embodiments will be now described herein with reference toillustrative embodiments. Those skilled in the art will recognize thatmany alternative embodiments can be accomplished using the teaching ofthe present invention and that the present invention is not limited tothe embodiments illustrated herein for explanatory purposes.

One or more embodiments provide a signal detecting device capable ofseparating and detecting amplitudes of a time-divisional signal of whichthe amplitude time-divisionally changes and an optical fibercharacteristics measuring device including the signal detecting device.

Hereinafter, a signal detecting device and an optical fibercharacteristics measuring device according to one or more embodimentswill be described in detail with reference to the drawings. Hereinafter,first, an overview of one or more embodiments will be described, andsubsequently, details of one or more embodiments will be described indetail.

[Overview]

One or more embodiments can separate and detect amplitudes of atime-divisional signal of which the amplitude time-divisionally changes.For example, in the optical fiber characteristics measuring device, in acase in which pulse light is caused to be sequentially incident in anoptical fiber, amplitudes of a time-divisional signal acquired bydetecting weak Brillouin scattering light sequentially output from theoptical fiber are separated and are caused to be detectable.

A lock-in amplifier extracts a DC component from a signal acquired bymultiplying a measurement signal by a reference signal using a low passfilter, thereby detecting a minute signal embedded in noise with highsensitivity. In the lock-in amplifier, in a case in which a component ofa frequency f included in the measurement signal is extracted, a signalof the frequency f is used as a reference signal. When the measurementsignal is multiplied by the reference signal, a DC component and asecond harmonic component (a 2f component) can be acquired. In aconventional lock-in amplifier, by extracting only the acquired DCcomponent using a low pass filter, a minute signal embedded in noise isdetected with high sensitivity. In addition, a second harmonic component(a 2f component) also can be acquired by using a high pass filter inplace of the low pass filter.

When a measurement signal including a time-divisional signal of which anamplitude time-divisionally changes is input to such a conventionallock-in amplifier, only one output acquired by performing low passfilter processing on a time-divisional signal can be acquired from theconventional lock-in amplifier. For this reason, each of amplitudes of atime-divisional signal cannot be separated and detected by theconventional lock-in amplifier.

A signal detecting device according to one or more embodiments includesa multiplier multiplying a measurement signal with a reference signal, afilter processor (a filter) performing filter processing on amultiplication result acquired by the multiplier, a first storagestoring an internal state of the filter processor, and a second storagestoring a processing result acquired by the filter processor. The filterprocessor performs filter processing using an internal state stored inthe first storage. The first storage performs switching of an areainto/from which an internal state is written/read by the filterprocessor in accordance with an index signal representing a type ofamplitude of a time-divisional signal included in a measurement signal.The second storage performs switching of an area in which a processingresult acquired by the filter processor is stored in accordance with anindex signal. In accordance with this, amplitudes of a time-divisionalsignal of which the amplitude time-divisionally changes can be separatedand detected.

[Signal detecting device]

First Example

FIG. 1 is a block diagram illustrating a main configuration of a signaldetecting device according to a first example. As illustrated in FIG. 1, the signal detecting device 1 according to this example includes anADC (an analog/digital converter) 11 (a first converter), an ADC 12 (asecond converter), a multiplier 13, a re-sampler 14, a change pointdetector 15 (a detector), a filter processor 16, an internal memory 17(a first storage), and a memory 18 (a second storage).

A measurement signal MS, a reference signal RS, and an index signal ISare input to the signal detecting device 1. The signal detecting device1 separates and detects amplitudes of the time-divisional signal DS (seeFIG. 2A) included in the measurement signal MS using the referencesignal RS and the index signal IS. In addition, the signal detectingdevice 1 includes a measurement signal input terminal T11 to which themeasurement signal MS is input, a reference signal input terminal T12 towhich the reference signal RS is input, an index signal input terminalT13 to which the index signal IS is input, and an output terminal T20 towhich a detected amplitude is output.

FIG. 2A to 2C are diagrams illustrating examples of signals input to thesignal detecting device according to the first example. FIG. 2A is adiagram illustrating an example of the measurement signal MS, FIG. 2B isa diagram illustrating an example of the reference signal RS, and FIG.2C is a diagram illustrating an example of the index signal IS. Asillustrated in FIG. 2A, the measurement signal MS is an analog signalincluding a time-divisional signal DS of which an amplitudetime-divisionally changes. The time-divisional signal DS illustrated inFIG. 2A is a signal of which an amplitude sequentially changes to A1,A2, and A3 for every two periods and is a weak signal, and thus a noiseis superimposed thereon. In addition, for example, the frequency of thetime-divisional signal DS is several to several tens of megahertz [MHz].

As illustrated in FIG. 2B, the reference signal RS is an analog signalthat has the same frequency as that of the time-divisional signal DSincluded in the measurement signal MS and is synchronized with thetime-divisional signal DS. As illustrated in FIG. 2C, the index signalIS is a signal that represents a type of amplitude of thetime-divisional signal DS included in the measurement signal MS and issynchronized with the time-divisional signal DS. The index signal ISillustrated in FIG. 2C represents that a type of amplitude A1 of thetime-divisional signal DS is “1”, a type of amplitude A2 of thetime-divisional signal DS is “2”, and a type of amplitude A3 of thetime-divisional signal DS is “3”. The index signal IS is a digitalsignal of a plurality of bits (for example, 16 bits).

In the examples illustrated in FIGS. 2A to 2C, for simplification ofdescription, although a case in which the amplitude of thetime-divisional signal DS is three types (amplitudes A1, A2, and A3) isillustrated as an example, the amplitude of the time-divisional signalDS may be two types or four or more types. In a case in which the indexsignal IS is of 16 bits, the number of types of amplitudes of thetime-divisional signal DS that can be detected by the signal detectingdevice 1 is a maximum of 65,536 types (=2¹⁶ types).

The ADC 11 converts a measurement signal MS input from the measurementsignal input terminal T11 into a digital signal. The ADC 12 converts areference signal RS input from the reference signal input terminal T12into a digital signal. The multiplier 13 multiplies the measurementsignal MS converted into the digital signal by the ADC 11 with thereference signal RS converted into the digital signal by the ADC 12. Forexample, the sampling frequency of the ADCs 11 and 12 is severalhundreds of megahertz [MHz].

When a timing signal TM is output from the change point detector 15, there-sampler 14 resamples a multiplication result acquired by themultiplier 13. Here, a first reason for disposing the re-sampler 14 isthat processing timings after the re-sampler 14 can be aligned attime-divisional timings of the time-divisional signal DS, and thusprocessing can be easily performed. A second reason for disposing there-sampler 14 is that a DC component included in a multiplication resultacquired by the multiplier 13 is finally detected by the signaldetecting device 1, and thus there is no problem even in a case in whicha high frequency component disappears in accordance with re-sampling.For example, a sampling frequency (a re-sampling frequency) of there-sampler 14 is several to several tens of megahertz [MHz]. Inaddition, the re-sampler 14 performs an aliasing noise countermeasuresuch as a low pass filter processing passing a frequency component offrequencies equal to or lower than ½ of the re-sampling frequency onoriginal data (a multiplication result acquired by the multiplier 13) orthe like and then performs data thinning.

The change point detector 15 detects a timing at which the index signalIS changes. More specifically, the change point detector 15 detects achange in the value of the index signal IS and, in a case in which achange thereof has been detected, outputs a timing signal TM at thattiming. As described above, the re-sampler 14 resamples a multiplicationresult acquired by the multiplier 13 when the timing signal TM is outputfrom the change point detector 15. For this reason, the re-sampler 14can be regarded to re-sample a multiplication result acquired by themultiplier 13 at a timing detected by the change point detector 15.

The filter processor 16 performs filter processing on a multiplicationresult, which has been acquired by the multiplier 13, re-sampled by there-sampler 14. For example, as the filter processing described above,the filter processor 16 performs a process using an infinite impulseresponse (IIR) low pass filter. In addition, as the filter processingdescribed above, the filter processor 16 may perform a process using afinite impulse response (FIR) low pass filter. In a case in which theinfinite impulse response low pass filter is used, an internal statequantity can be configured to be smaller than that in a case in whichthe finite impulse response low pass filter is used. For this reason,when a processing load of the filter processor 16 and the capacity ofthe internal memory 17 are considered, it is more preferable to use theinfinite impulse response low pass filter than the finite impulseresponse low pass filter.

The internal memory 17 is a memory that stores an internal state of thefilter processor 16. The internal memory 17 performs switching of anarea of which an internal state is written and read using the filterprocessor 16 in accordance with the index signal IS. The memory 18 is amemory that stores a processing result acquired by the filter processor16. The memory 18 performs switching of an area in which the processingresult acquired by the filter processor 16 is stored in accordance withthe index signal IS.

FIGS. 3A and 3B are diagrams illustrating examples of a memory map ofthe signal detecting device according to the first example. FIG. 3A is adiagram illustrating a memory map of the internal memory 17, and FIG. 3Bis a diagram illustrating a memory map of the memory 18. As illustratedin FIGS. 3A and 3B, in each of the internal memory 17 and the memory 18,three areas are disposed in accordance with types of amplitude(amplitudes A1, A2, and A3) of the time-divisional signal DS.

An area R11 illustrated in FIG. 3A, for example, is an area in/fromwhich an internal state is written/read by the filter processor 16 in acase in which the index signal IS is “1”. An area R12, for example, isan area in/from which an internal state is written/read by the filterprocessor 16 in a case in which the index signal IS is “2”. An area R13,for example, is an area in/from which an internal state is written/readby the filter processor 16 in a case in which the index signal IS is“3”.

An area R21 illustrated in FIG. 3B, for example, is an area in which aprocessing result acquired by the filter processor 16 is stored in acase in which the index signal IS is “1”. An area R22, for example, isan area in which a processing result acquired by the filter processor 16is stored in a case in which the index signal IS is “2”. An area R23,for example, is an area in which a processing result acquired by thefilter processor 16 is stored in a case in which the index signal IS is“3”.

In other words, in a case in which filter processing for thetime-divisional signal DS of which an amplitude is amplitude A1illustrated in FIG. 2A is performed, an internal state stored in thearea R11 illustrated in FIG. 3A is used, and a processing result isstored in the area R21 illustrated in FIG. 3B. In a case in which filterprocessing for the time-divisional signal DS of which an amplitude isamplitude A2 illustrated in FIG. 2A is performed, an internal statestored in the area R12 illustrated in FIG. 3A is used, and a processingresult is stored in the area R22 illustrated in FIG. 3B. In a case inwhich filter processing for the time-divisional signal DS of which anamplitude is amplitude A3 illustrated in FIG. 2A is performed, aninternal state stored in the area R13 illustrated in FIG. 3A is used,and a processing result is stored in the area R23 illustrated in FIG.3B.

As illustrated in FIGS. 3A and 3B, the number of areas disposed in eachof the internal memory 17 and the memory 18 is a number corresponding totypes of amplitudes of the time-divisional signal DS. In a case in whichthe types of amplitude of the time-divisional signal DS are huge, thenumber of areas disposed in the internal memory 17 and the memory 18also become huge. In such a case, it can be handled by increasing thecapacity of each of the internal memory 17 and the memory 18.

Next, an operation of the signal detecting device 1 in the configurationdescribed above will be described. When the operation of the signaldetecting device 1 starts, a measurement signal MS, a reference signalRS, and an index signal IS are respectively input from the measurementsignal input terminal T11, the reference signal input terminal T12, andthe index signal input terminal T13. The time-divisional signal DS, thereference signal RS, and the index signal IS included in the measurementsignal MS are synchronized with each other.

The measurement signal MS input from the measurement signal inputterminal T11 is converted into a digital signal by the ADC 11, and thereference signal RS input from the reference signal input terminal T12is converted into a digital signal by the ADC 12. The measurement signalMS and the reference signal RS converted into the digital signals aremultiplied with each other by the multiplier 13.

On the other hand, the index signal IS input from the index signal inputterminal T13 is input to the change point detector 15, and a change inthe value thereof is detected. When a change in the value is detected bythe change point detector 15, a timing signal TM is output from thechange point detector 15 to the re-sampler 14. In addition, the indexsignal IS is input to the internal memory 17 and the memory 18, and anarea for storing an internal state of the filter processor 16 and anarea for storing a processing result acquired by the filter processor 16are set in each thereof.

For example, it is assumed that the input index signal IS input from theindex signal input terminal T13 is “1”. At this time, as an area forstoring the internal state of the filter processor 16, for example, thearea R11 of the internal memory 17 illustrated in FIG. 3A is set. Inaddition, as an area for storing a processing result acquired by thefilter processor 16, for example, the area R21 of the memory 18illustrated in FIG. 3B is set.

A multiplication result acquired by the multiplier 13 is input to there-sampler 14 and is re-sampled with a timing of the timing signal TMoutput from the change point detector 15. The multiplication result,which has been acquired by the multiplier 13, re-sampled by there-sampler 14 is input to the filter processor 16, and filter processingis performed thereon. More specifically, the filter processor 16performs a process of extracting a DC component by performing a processusing an infinite impulse response low pass filter.

In a case in which an internal state is stored in an area (here, thearea R11 illustrated in FIG. 3A) of the internal memory 17 set inaccordance with the index signal IS, the filter processor 16 reads theinternal state and performs the filter processing described above.Internal states of the filter processor 16 are sequentially written intoan area (here, the area R11 illustrated in FIG. 3A) of the internalmemory 17 set in accordance with the index signal IS.

A processing result (an extracted DC component) acquired by the filterprocessor 16 is stored in an area (here, the area R21 illustrated inFIG. 3B) of the memory 18 set in accordance with the index signal IS.The DC component extracted here represents a magnitude of the amplitudeA1 illustrated in FIG. 2A.

Next, the index signal IS input from the index signal input terminal T13is assumed to change from “1” to “2”. Then, the change of the value isdetected by the change point detector 15, and a timing signal TM isoutput from the change point detector 15 to the re-sampler 14. Then, inthe re-sampler 14, a multiplication result acquired by the multiplier 13is re-sampled with the timing of the timing signal TM.

In addition, in accordance with the change of the index signal IS, thearea for storing the internal state of the filter processor 16 and thearea for storing a processing result acquired by the filter processor 16are newly set. More specifically, in a case in which the index signal IShas changed to “2”, as the area for storing the internal state of thefilter processor 16, for example, the area R12 of the internal memory 17illustrated in FIG. 3A is newly set. In addition, as the area forstoring the processing result acquired by the filter processor 16, forexample, the area R22 of the memory 18 illustrated in FIG. 3B is newlyset.

The multiplication result, which has been acquired by the multiplier 13,re-sampled by the re-sampler 14 is input to the filter processor 16, andfilter processing is performed thereon. Here, in a case in which theinternal state is stored in an area (hereinafter, the area R12illustrated in FIG. 3A) of the internal memory 17 set in accordance withthe index signal IS, the filter processor 16 reads the internal stateand performs the filter processing described above. In addition,internal states of the filter processor 16 are sequentially written intoan area (here, the area R12 illustrated in FIG. 3A) of the internalmemory 17 set in accordance with the index signal IS.

A processing result (an extracted DC component) acquired by the filterprocessor 16 is stored in an area (here, the area R22 illustrated inFIG. 3B) of the memory 18 set in accordance with the index signal IS.The DC component extracted here represents a magnitude of the amplitudeA2 illustrated in FIG. 2A.

Next, the index signal IS input from the index signal input terminal T13is assumed to change from “2” to “3”. Then, the change of the value isdetected by the change point detector 15, and a timing signal TM isoutput from the change point detector 15 to the re-sampler 14. Then, inthe re-sampler 14, a multiplication result acquired by the multiplier 13is re-sampled with the timing of the timing signal TM.

In addition, in accordance with the change of the index signal IS, thearea for storing the internal state of the filter processor 16 and thearea for storing a processing result acquired by the filter processor 16are newly set. More specifically, in a case in which the index signal IShas changed to “3”, as the area for storing the internal state of thefilter processor 16, for example, the area R13 of the internal memory 17illustrated in FIG. 3A is newly set. In addition, as the area forstoring the processing result acquired by the filter processor 16, forexample, the area R23 of the memory 18 illustrated in FIG. 3B is newlyset.

The multiplication result, which has been acquired by the multiplier 13,re-sampled by the re-sampler 14 is input to the filter processor 16, andfilter processing is performed thereon. Here, in a case in which theinternal state is stored in an area (hereinafter, the area R13illustrated in FIG. 3A) of the internal memory 17 set in accordance withthe index signal IS, the filter processor 16 reads the internal stateand performs the filter processing described above. In addition,internal states of the filter processor 16 are sequentially written intoan area (here, the area R13 illustrated in FIG. 3A) of the internalmemory 17 set in accordance with the index signal IS.

A processing result (an extracted DC component) acquired by the filterprocessor 16 is stored in an area (here, the area R23 illustrated inFIG. 3B) of the memory 18 set in accordance with the index signal IS.The DC component extracted here represents a magnitude of the amplitudeA3 illustrated in FIG. 2A.

Hereinafter, similarly, when the value of the index signal IS changes, amultiplication result acquired by the multiplier 13 is re-sampled withthe timing, and an area for storing the internal state of the filterprocessor 16 and an area for storing a processing result acquired by thefilter processor 16 are set. Here, in a case in which an area accordingto the value of the index signal IS has been set in advance, the area isset as an area for storing the internal state of the filter processor 16and an area for storing a processing result acquired by the filterprocessor 16. On the other hand, in a case in which an area according tothe value of the index signal IS has not been set, an area for storingthe internal state of the filter processor 16 and an area for storing aprocessing result acquired by the filter processor 16 are newly set.

For example, the value of the index signal IS is assumed to change to“1” again. Then, for example, the area R11 of the internal memory 17illustrated in FIG. 3A and the area R21 of the memory 18 illustrated inFIG. 3B are respectively set as an area for storing the internal stateof the filter processor 16 and an area for storing a processing resultacquired by the filter processor 16. On the other hand, for example, thevalue of the index signal IS is assumed to become a new value “4”. Then,as an area for storing the internal state of the filter processor 16,for example, an area other than the areas R11 to R13 of the internalmemory 17 illustrated in FIG. 3A is newly set. In addition, as an areafor storing a processing result acquired by the filter processor 16, forexample, an area other than the areas R21 to R23 of the memory 18illustrated in FIG. 3B is newly set.

By performing the process described above, values representingmagnitudes of the amplitudes A1, A2, and A3 of the time-divisionalsignal DS illustrated in FIG. 2A are stored in mutually-different areasof the memory 18. For example, a value representing a magnitude of theamplitude A1 is stored in the area R21 of the memory 18, a valuerepresenting the magnitude of the amplitude A2 is stored in the area R22of the memory 18, and a value representing the magnitude of theamplitude A3 is stored in the area R23 of the memory 18 (see FIG. 3B).

For example, each of values stored in mutually-different areas of thememory 18 can be read by designating a read address of the memory 18. Avalue read from the memory 18 is output from the output terminal T20 ofthe signal detecting device 1. In this way, each of amplitudes of atime-divisional signal DS included in a measurement signal MS areseparated and detected.

As described above, this example includes the multiplier 13 thatmultiplies a measurement signal MS by a reference signal RS and thefilter processor 16 that performs filter processing on a multiplicationresult acquired by the multiplier 13. In addition, this example includesthe internal memory 17 that stores an internal state of the filterprocessor 16 and the memory 18 that stores a processing result acquiredby the filter processor 16. Then, the filter processor 16 performsfilter processing using the internal state stored in the internal memory17. The internal memory 17 performs switching of an area in/from whichan internal state is written/read using the filter processor 16 inaccordance with an index signal IS representing a type of amplitude of atime-divisional signal DS included in a measurement signal MS. Thememory 18 performs switching of an area in which a processing resultacquired by the filter processor 16 is stored in accordance with theindex signal IS. In accordance with this, each of amplitudes of atime-divisional signal DS of which the amplitude time-divisionallychanges can be separated and detected.

Second Example

FIG. 4 is a block diagram illustrating a main configuration of a signaldetecting device according to a second example. In FIG. 4 , the samereference signs will be assigned to the same blocks as the blocksillustrated in FIG. 1 . As illustrated in FIG. 4 , the signal detectingdevice 2 according to this example has a configuration in which the ADC12 and the change point detector 15 of the signal detecting device 1illustrated in FIG. 1 are omitted, and a reference signal generator 21(a first generator) and an index signal generator 22 (a secondgenerator) are added.

A measurement signal MS and a start trigger signal TS are input to thesignal detecting device 2. The start trigger signal TS is a signal thatdefines a start timing of a time-divisional signal DS included in ameasurement signal MS. The signal detecting device 2 generates areference signal RS and an index signal IS from the start trigger signalTS and separates and detects amplitudes of the time-divisional signal DS(see FIG. 2A) included in the measurement signal MS using the referencesignal RS and the index signal IS that have been generated.

The signal detecting device 2 according to this example has such apremise that a frequency of the time-divisional signal DS, a period atwhich the amplitude of the time-divisional signal DS changes, and typesof amplitude of the time-divisional signal DS are known. In the signaldetecting device 2, the reference signal input terminal T12 and theindex signal input terminal T13 illustrated in FIG. 1 are omitted, and astart trigger signal input terminal T14 to which the start triggersignal TS is input is included.

The reference signal generator 21 starts generation of a referencesignal RS at a timing at which the start trigger signal TS is input fromthe start trigger signal input terminal T14. The reference signal RSgenerated by the reference signal generator 21 is a digital signalhaving the same frequency as the frequency of the time-divisional signalDS that is known. After the start trigger signal TS is input, thereference signal generator 21 generates a timing signal TM every time aperiod with which the amplitude of the time-divisional signal DS that isknown changes elapses. This timing signal TM is a signal representing adivision timing of the time-divisional signal DS and is input to there-sampler 14 and the index signal generator 22. In addition, the timingsignal TM can be regarded as a signal similar to the timing signal TMoutput from the change point detector 15 in the first example.

The index signal generator 22 generates an index signal IS on the basisof the timing signal TM generated by the reference signal generator 21.For example, a case in which the number of types of amplitude of thetime-divisional signal DS, which is known, is “3” will be described. Inthis case, similar to the index signal IS illustrated in FIG. 2C, when atiming signal TM is output from the reference signal generator 21, theindex signal generator 22 generates an index signal IS changing to “1”,“2”, “3”, “1”, “2”, . . . .

An operation of the signal detecting device 2 in the configuration isbasically similar to the operation of the signal detecting device 1according to the first example except that a reference signal RS and anindex signal IS are generated inside the signal detecting device 2 onthe basis of the start trigger signal TS. For this reason, here, theoperation of the signal detecting device 2 will be briefly described.

When the operation of the signal detecting device 2 starts, ameasurement signal MS and a start trigger signal TS are respectivelyinput from the measurement signal input terminal T11 and the starttrigger signal input terminal T14. The time-divisional signal DSincluded in the measurement signal MS and the start trigger signal TSare synchronized with each other.

The measurement signal MS input from the measurement signal inputterminal T11 is converted into a digital signal by the ADC 11. The starttrigger signal TS input from the start trigger signal input terminal T14is input to the reference signal generator 21. In accordance with this,in the reference signal generator 21, generation of a reference signalRS as a digital signal starts at a timing at which the start triggersignal TS is input. The measurement signal MS converted into the digitalsignal is multiplied by the reference signal RS generated by thereference signal generator 21 using the multiplier 13.

In addition, after the start trigger signal TS is input, every time aperiod in which the amplitude of the time-divisional signal DS changeselapses, a timing signal TM is generated by the reference signalgenerator 21. This timing signal TM is input to the re-sampler 14 and isinput to the index signal generator 22. When the timing signal TM isinput to the re-sampler 14, a multiplication result acquired by themultiplier 13 is re-sampled with a timing of the timing signal TM by there-sampler 14. On the other hand, when the timing signal TM is input tothe index signal generator 22, an index signal IS is generated by theindex signal generator 22.

The multiplication result, which has been acquired by the multiplier 13,re-sampled by the re-sampler 14 is input to the filter processor 16, andfilter processing is performed thereon. On the other hand, the indexsignal IS generated by the index signal generator 22 is input to theinternal memory 17 and the memory 18, and, similar to the first example,an area for storing the internal state of the filter processor 16 and anarea for storing the processing result acquired by the filter processor16 are set.

When a timing signal TM is generated by the reference signal generator21, the value of the index signal IS generated by the index signalgenerator 22 changes. In accordance with this, the area for storing theinternal state of the filter processor 16 in the internal memory 17 andthe area for storing the processing result acquired by the filterprocessor 16 in the memory 18 are sequentially switched. By performingsuch a process, similar to the first example, values representingmagnitudes of the amplitudes A1, A2, and A3 of the time-divisionalsignal DS illustrated in FIG. 2A are stored in mutually-different areasof the memory 18.

Also in this example, for example, each of the values stored in thedifferent areas of the memory 18 can be read by designating a readaddress of the memory 18. The value read from the memory 18 is outputfrom an output terminal T20 of the signal detecting device 2. In thisway, amplitudes of the time-divisional signal DS included in themeasurement signal MS are separated and detected.

As described above, this example includes the multiplier 13 thatmultiplies a measurement signal MS by a reference signal RS and thefilter processor 16 that performs filter processing on a multiplicationresult acquired by the multiplier 13. In addition, this example includesthe internal memory 17 that stores an internal state of the filterprocessor 16 and the memory 18 that stores a processing result acquiredby the filter processor 16. Furthermore, this example includes thereference signal generator 21 that generates a reference signal RS onthe basis of the start trigger signal TS and generates a timing signalTM and the index signal generator 22 that generates an index signal ISon the basis of the timing signal TM.

Then, the filter processor 16 performs filter processing using theinternal state stored in the internal memory 17. The internal memory 17performs switching of an area of which an internal state is written andread using the filter processor 16 in accordance with the index signalIS generated by the index signal generator 22. The memory 18 performsswitching of an area in which the processing result acquired by thefilter processor 16 is stored in accordance with the index signal ISgenerated by the index signal generator 22. In accordance with this,amplitudes of the time-divisional signal DS of which the amplitudetime-divisionally changes can be separated and detected.

In addition, in this example, a reference signal RS and an index signalIS are generated inside the signal detecting device 2 on the basis ofthe start trigger signal TS input from the outside. In accordance withthis, the number of signals input to the signal detecting device 2 canbe reduced from the number of signals input to the signal detectingdevice 1, and a noise and a jitter can be reduced. Such a signaldetecting device 2 can be expected to exhibit performance higher thanthat of the signal detecting device 1.

[Optical Fiber Characteristics Measuring Device]

FIG. 5 is a block diagram illustrating a main configuration of anoptical fiber characteristics measuring device according to one or moreembodiments. As illustrated in FIG. 5 , the optical fibercharacteristics measuring device MD according to this example includes asignal generator 30, a light source 31, an optical splitter 32 (a firstoptical splitter), a pulsator 33, a light delayer 34, a light amplifier35, an optical splitter 36 (a second optical splitter), a lightamplifier 37, an optical combiner 38, a light detector 39 (a firstdetector), a frequency analyzer 40 (an analyzer), a second harmoniccomponent detector 41 (a second detector), and a measurer 42.

The optical fiber characteristics measuring device MD according to thisexample is an optical fiber characteristics measuring device using aso-called Brillouin optical correlation domain reflectometry (BOCDR)method measuring characteristics of an optical fiber under test FUT onthe basis of Brillouin scattering light LS acquired by causing pumppulse light P to be incident in the optical fiber under test FUT. Inaddition, the pump pulse light P described above is acquired byperforming pulsation of pump light LP as continuous light for whichfrequency modulation has been performed. The Brillouin scattering lightLS is rear-side scattering light generated in accordance with Brillouinscattering inside an optical fiber under test FUT.

As the optical fiber under test FUT, an arbitrary optical fiber can beused in accordance with a wavelength and the like of pump pulse light P.In this example, it is assumed that a length of the optical fiber undertest FUT is longer than an interval dm of correlation peaks, and aplurality of correlation peaks are present in the optical fiber FUTunder test.

The signal generator 30 generates a modulation signal Sm supplied to thelight source 31, a pulsation signal Sp supplied to the pulsator 33, anda start trigger signal TS supplied to the second harmonic componentdetector 41. The modulation signal Sm is a signal used for outputtingcontinuous light L1 (modulation light) for which frequency modulated hasbeen performed from the light source 31. A frequency (a modulationfrequency fm) of the modulation signal Sm is swept in a frequency rangedefined in advance. The pulsation signal Sp is a signal for pulsatingthe pump light LP as continuous light. The start trigger signal TS is asignal that causes the second harmonic component detector 41 to startdetection of a second harmonic component included in a Brillouin gainspectrum acquired by the frequency analyzer 40.

The light source 31 includes a light source 31 a and a drive signalgenerator 31 b and outputs continuous light L1 for which frequencymodulation has been performed using a modulation signal Sm output fromthe signal generator 30. The light source 31 a, for example, includes asemiconductor laser element such as a distributed feed-back laser diode(DFB-LD) and outputs continuous light L1 for which frequency modulationhas been performed in accordance with a drive signal D1 output from thedrive signal generator 31 b. The drive signal generator 31 b generates adrive signal D1 used for outputting the frequency-modulated continuouslight L1 from the light source 31 a using the modulation signal Smoutput from the signal generator 30.

The optical splitter 32 splits continuous light L1 output from the lightsource 31 into pump light LP and reference light LR having an intensityratio defined in advance (for example, 1:1). The pulsator 33 pulsatespump light LP split by the first optical splitter 32 using a pulsationsignal Sp output from the signal generator 30. The reason for disposingthe pulsator 33 is to acquire pump light P used in a time gate method.

The light delayer 34 delays pump light LP formed as a pulse (pump pulselight P) by the pulsator 33 by a predetermined time. The light delayer34, for example, includes an optical fiber of a predetermined length. Bychanging the length of the optical fiber, the delay time can beadjusted. The reason for disposing the light delayer 34 is to dispose a0-order correlation peak, of which an appearing position does not moveeven when sweeping of the modulation frequency fm is performed, outsidethe optical fiber under test FUT.

The light amplifier 35 amplifies the pump pulse light P through thelight delayer 34. This light amplifier 35, for example, includes a lightamplifier such as an Erbium doped fiber amplifier (EDFA) and amplifiesthe pump pulse light P with a predetermined amplification factor.

The optical splitter 36 includes a first port, a second port, and athird port. The first port is connected to the light amplifier 35. Thesecond port is connected to the optical fiber under test FUT. The thirdport is connected to the light amplifier 37. The optical splitter 36outputs pump pulse light P input from the first port to the second port.In addition, the optical splitter 36 outputs Brillouin scattering lightLS from the optical fiber under test FUT, which is input from the secondport, to the third port. As such an optical splitter 36, for example, anoptical circulator can be used.

The light amplifier 37 amplifies Brillouin scattering light LS outputfrom the third port of the optical splitter 36. Similar to the lightamplifier 35, this light amplifier 37, for example, includes a lightamplifier such as an EDFA and amplifies the Brillouin scattering lightLS output from the third port of the optical splitter 36 with apredetermined amplification factor.

The optical combiner 38 combines the Brillouin scattering light LSamplified by the light amplifier 37 and the reference light LR split bythe optical splitter 32. In addition, the optical combiner 38 splits thecombined light into two pieces of light having an intensity ratio (forexample, 1:1) defined in advance and outputs the light to the lightdetector 39. The two pieces of light split by the optical combiner 38,for example, includes 50% of rear-side scattering light from the opticalfiber under test FUT and 50% of reference light. As such an opticalcombiner 38, for example, an optical coupler can be used.

The light detector 39 causes the Brillouin scattering light LS and thereference light LR included in two pieces of light output from theoptical combiner 38 to interfere with each other, thereby performingoptical heterodyne detection. The light detector 39, for example,includes a balanced photodiode formed from two photo diodes (PDs) 39 aand 39 b and an adder 39 c. The photodiodes 39 a and 39 b receive twopieces of light output from the optical combiner 38. Light receptionsignals of the photodiodes 39 a and 39 b are input to the adder 39 c.From the adder 39 c, a detection signal S1 that is an interventionsignal (a beat signal) representing a frequency difference between theBrillouin scattering light LS and the reference light LR is output.

The frequency analyzer 40 performs a frequency analysis of the detectionsignal S1 output from the light detector 39. In other words, thefrequency analyzer 40 acquires a Brillouin gain spectrum from thedetection signal S1 output from the light detector 39. The frequencyanalyzer 40, for example, includes a spectrum analyzer (ElectricalSpectrum analyzer (ESA)). The frequency analyzer 40 takes in thedetection signal S1 output from the light detector 39 during a perioddefined using a time gate method. In accordance with this, even when aplurality of correlation peaks is present in the optical fiber undertest FUT, characteristics of the optical fiber under test FUT can bemeasured without any problem.

In addition, the frequency analyzer 40 may be configured to include atime axis measurer such as an oscilloscope and a transformer performinga fast Fourier transform (FFT) in place of the spectrum analyzer. Thefrequency analyzer 40 having such a configuration transforms data, whichis continuous in time, acquired by the time axis measurer into spectrumdata using the transformer.

The second harmonic component detector 41 converts a Brillouin gainspectrum output from the frequency analyzer 40 into a digital signal andthen detects a second harmonic component included in the Brillouin gainspectrum. Here, the second harmonic component is a component having afrequency (2 fm) that is twice the modulation frequency fm of continuouslight L1. The reason for detecting such a second harmonic component isto perform stable measurement in a time shorter than that of aconventional case by eliminating a noise superimposed in the Brillouingain spectrum acquired by the frequency analyzer 40 and inhibitingvariations of a baseline.

This second harmonic component represents an intensity at the frequency(2 fm) of the Brillouin gain spectrum acquired by the frequency analyzer40 and has a magnitude changing in accordance with a position of theoptical fiber under test FUT at which the Brillouin gain spectrum hasbeen acquired. For example, in a case in which the Brillouin gainspectrum acquired by the frequency analyzer 40 is acquired at a positionat which a correlation peak appears, the magnitude of the secondharmonic component is the largest.

The second harmonic component detector 41 includes the signal detectingdevice 2 illustrated in FIG. 4 . A Brillouin gain spectrum acquired bythe frequency analyzer 40 is input to the signal detecting device 2 as ameasurement signal MS, and a start trigger signal TS output from thesignal generator 30 is also input to the signal detecting device 2. Whenthe start trigger signal TS is input, the signal detecting device 2generates a reference signal RS having a frequency that is twice themodulation frequency of modulation light and an index signal IS anddetects a second harmonic component.

The measurer 42 measures characteristics of the optical fiber under testFUT on the basis of the Brillouin gain spectrum that has been convertedinto a digital signal by the second harmonic component detector 41 andthe second harmonic component detected by the second harmonic componentdetector 41. More specifically, the measurer 42 acquires a peakfrequency of the Brillouin gain spectrum by performing digitalprocessing using the Brillouin gain spectrum that has been convertedinto a digital signal by the second harmonic component detector 41 andthe second harmonic component detected by the second harmonic componentdetector 41. Then, a Brillouin frequency shift amount is acquired fromthe acquired peak frequency, and this Brillouin frequency shift amountis converted into a magnitude of distortion and a temperature changeapplied to the optical fiber under test FUT. In addition, the measurer42 may include a display that displays a second harmonic componentdetected by the second harmonic component detector 41, characteristicsof the measured optical fiber under test FUT (for example, a distortiondistribution), and the like. For example, the display is a liquidcrystal display, an organic electroluminescence (EL) display device, orthe like.

In this example, first, continuous light L1 for which frequencymodulation has been performed is split into pump light LP and referencelight LR, the pump light LP is transformed into pump pulse light P,then, the pump pulse light is incident from one end of the optical fiberunder test FUT, and Brillouin scattering light LS generated inside theoptical fiber under test is acquired. Next, interference light betweenthe Brillouin scattering light LS and the reference light LR isdetected, and a Brillouin gain spectrum that is a spectrum of theBrillouin scattering light is acquired. Then, a second harmoniccomponent having a frequency that is twice the modulation frequency ofthe modulation light included in the acquired Brillouin gain spectrum isdetected by the signal detecting device, and characteristics of theoptical fiber under test are measured on the basis of the acquiredBrillouin gain spectrum and the detected second harmonic component. Inaccordance with this, stable measurement can be performed in a timeshorter than that of a conventional case.

As above, although the signal detecting device and the optical fibercharacteristics measuring device according to embodiments have beendescribed, the present invention is not limited to the embodimentsdescribed above, and changes can be freely made within the scope of thepresent invention. For example, the second harmonic component detector41 (see FIG. 5 ) of the optical fiber characteristics measuring deviceMD described in the embodiments described above is configured to includethe signal detecting device 2 according to the second exampleillustrated in FIG. 4 . However, the second harmonic component detector41 may be configured to include the signal detecting device 1 accordingto the first example illustrated in FIG. 1 .

[Supplementary Note]

A signal detecting device (1, 2) of a first aspect of one or moreembodiments may include: a multiplier (13) configured to multiply ameasurement signal (MS) by a reference signal (RS); a filter processor(16) configured to perform filter processing on a multiplication resultacquired by the multiplier; a first storage (17) configured to store aninternal state of the filter processor; and a second storage (18)configured to store a processing result acquired by the filterprocessor. The filter processor may be configured to perform the filterprocessing using the internal state stored in the first storage. Thefirst storage may be configured to perform switching of an area in/fromwhich the internal state is written/read by the filter processor inaccordance with an index signal (IS) representing a type of amplitude ofa time-divisional signal (DS) included in the measurement signal. Thesecond storage may be configured to perform switching of an area inwhich the processing result acquired by the filter processor is storedin accordance with the index signal.

In addition, the signal detecting device of a second aspect of one ormore embodiments according to the first aspect may further include: afirst converter (11) configured to convert the measurement signal into adigital signal; a second converter (12) configured to convert thereference signal into a digital signal; a detector (15) configured todetect a timing at which the index signal changes; and a re-sampler (14)configured to re-sample the multiplication result acquired by themultiplier at a timing detected by the detector.

Furthermore, the signal detecting device of a third aspect of the one ormore embodiments according to the first or second aspect may furtherinclude: a measurement signal input terminal (T11) to which themeasurement signal is input; a reference signal input terminal (T12) towhich the reference signal is input; and an index signal input terminal(T13) to which the index signal is input.

In addition, the signal detecting device of a fourth aspect of one ormore embodiments according to the first aspect may further include: afirst generator (21) configured to generate the reference signal on thebasis of a start trigger signal (TS) defining a start timing of thetime-divisional signal, and configured to generate a timing signal (TM)representing a division timing of the time-divisional signal; and asecond generator (22) configured to generate the index signal on thebasis of the timing signal.

Furthermore, in the signal detecting device of a fifth aspect of one ormore embodiments according to the fourth aspect, the first generator isconfigured to generate the reference signal as a digital signal, and thesignal detecting device may further include: a first converter (11)configured to convert the measurement signal into a digital signal; anda re-sampler (14) configured to re-sample the multiplication resultacquired by the multiplier at a division timing represented in thetiming signal.

In addition, the signal detecting device of a sixth aspect of one ormore embodiments according to the fourth or fifth aspect may furtherinclude: a measurement signal input terminal (T11) to which themeasurement signal is input; and a start trigger signal input terminal(T14) to which the start trigger signal is input.

Furthermore, in the signal detecting device of a seventh aspect of oneor more embodiments according to any one of the first to sixth aspects,the filter processor may be configured to perform a process using aninfinite impulse response low pass filter as the filter processing.

In addition, in the signal detecting device of an eighth aspect of oneor more embodiments according to first aspect, the first storage may beconfigured to set a new area as an area for storing the internal stateof the filter processor when a value of the index signal is a new value.

In addition, in the signal detecting device of a ninth aspect of one ormore embodiments according to first aspect, the second storage may beconfigured to set a new area as an area for storing the processingresult acquired by the filter processor when a value of the index signalis a new value.

In addition, in the signal detecting device of a tenth aspect of one ormore embodiments according to fourth aspect, after the start triggersignal is input, the first generator may be configured to generate thetiming signal every time a period with which the amplitude of thetime-divisional signal changes elapses.

An optical fiber characteristics measuring device (MD) of an eleventhaspect of one or more embodiments may include: a first optical splitter(32) configured to split modulation light (L1) for which frequencymodulation has been performed into pump light (LP) and reference light(LR); a second optical splitter (36) configured to cause the pump lightto be incident from one end of an optical fiber under test (FUT), andconfigured to output Brillouin scattering light (LS) generated insidethe optical fiber under test; a first detector (39) configured to detectinterference light between the Brillouin scattering light output fromthe second optical splitter and the reference light; an analyzer (40)configured to acquire a Brillouin gain spectrum that is a spectrum ofthe Brillouin scattering light from a detection signal (S1) output fromthe first detector; a second detector (41) configured to detect a secondharmonic component having a frequency that is twice a modulationfrequency of the modulation light included in the Brillouin gainspectrum acquired by the analyzer; and a measurer (42) configured tomeasure characteristics of the optical fiber under test on the basis ofthe Brillouin gain spectrum acquired by the analyzer and the secondharmonic component detected by the second detector. The second detectormay include the signal detecting device according to any one of thefirst to tenth aspects to which the Brillouin gain spectrum acquired bythe analyzer is input as the measurement signal and which detects thesecond harmonic component using the reference signal having thefrequency that is twice the modulation frequency of the modulationlight.

In addition, in the optical fiber characteristics measuring device of atwelfth aspect of one or more embodiments according to the eleventhaspect, the signal detecting device may further include: a firstconverter (11) configured to convert the measurement signal into adigital signal; a second converter (12) configured to convert thereference signal into a digital signal; a detector (15) configured todetect a timing at which the index signal changes; and a re-sampler (14)configured to re-sample the multiplication result acquired by themultiplier at a timing detected by the detector.

Furthermore, in the optical fiber characteristics measuring device of athirteenth aspect of one or more embodiments according to the eleventhor twelfth aspect, the signal detecting device may further include: ameasurement signal input terminal (T11) to which the measurement signalis input; a reference signal input terminal (T12) to which the referencesignal is input; and an index signal input terminal (T13) to which theindex signal is input.

In addition, in the optical fiber characteristics measuring device of afourteenth aspect of one or more embodiments according to the eleventhaspect, the signal detecting device may further include: a firstgenerator (21) configured to generate the reference signal on the basisof a start trigger signal (TS) defining a start timing of thetime-divisional signal, and configured to generate a timing signal (TM)representing a division timing of the time-divisional signal; and asecond generator (22) configured to generate the index signal on thebasis of the timing signal.

Furthermore, in the optical fiber characteristics measuring device of afifteenth aspect of one or more embodiments according to the fourteenthaspect, the first generator is configured to generate the referencesignal as a digital signal, and the signal detecting device may furtherinclude: a first converter (11) configured to convert the measurementsignal into a digital signal; and a re-sampler (14) configured tore-sample the multiplication result acquired by the multiplier at adivision timing represented in the timing signal.

In addition, in the optical fiber characteristics measuring device of asixteenth aspect of one or more embodiments according to the fourteenthor fifteenth aspect, the signal detecting device may further include: ameasurement signal input terminal (T11) to which the measurement signalis input; and a start trigger signal input terminal (T14) to which thestart trigger signal is input.

Furthermore, in the optical fiber characteristics measuring device of aseventeenth aspect of one or more embodiments according to any one ofthe eleventh to sixteenth aspects, the filter processor may beconfigured to perform a process using an infinite impulse response lowpass filter as the filter processing.

In addition, in the optical fiber characteristics measuring device of aneighteenth aspect of one or more embodiments according to eleventhaspect, the first storage may be configured to set a new area as an areafor storing the internal state of the filter processor when a value ofthe index signal is a new value.

In addition, in the optical fiber characteristics measuring device of anineteenth aspect of one or more embodiments according to eleventhaspect, the second storage may be configured to set a new area as anarea for storing the processing result acquired by the filter processorwhen a value of the index signal is a new value.

In addition, in the optical fiber characteristics measuring device of atwentieth aspect of one or more embodiments according to fourteenthaspect, after the start trigger signal is input, the first generator maybe configured to generate the timing signal every time a period withwhich the amplitude of the time-divisional signal changes elapses.

According to one or more embodiments, there is an advantage of beingable to separate and detect amplitudes of a time-divisional signal ofwhich the amplitude time-divisionally changes.

As used herein, the following directional terms “front, back, above,downward, right, left, vertical, horizontal, below, transverse, row andcolumn” as well as any other similar directional terms refer to thoseinstructions of a device equipped with one or more embodiments.Accordingly, these terms, as utilized to describe one or moreembodiments should be interpreted relative to a device equipped with oneor more embodiments.

The term “configured” is used to describe a component, unit or part of adevice includes hardware and/or software that is constructed and/orprogrammed to carry out the desired function.

Moreover, terms that are expressed as “means-plus function” in theclaims should include any structure that can be utilized to carry outthe function of that part of the present invention.

The term “unit” is used to describe a component, unit or part of ahardware and/or software that is constructed and/or programmed to carryout the desired function. Typical examples of the hardware may include,but are not limited to, a device and a circuit.

While embodiments have been described and illustrated above, it shouldbe understood that these are examples of the present invention and arenot to be considered as limiting. Additions, omissions, substitutions,and other modifications can be made without departing from the scope ofthe present invention. Accordingly, the present invention is not to beconsidered as being limited by the foregoing description, and is onlylimited by the scope of the claims.

What is claimed is:
 1. A signal detecting device comprising: amultiplier that multiplies a measurement signal by a reference signal; afilter that filters a multiplication result from the multiplier; a firststorage that stores an internal state of the filter; and a secondstorage that stores a filtering result from the filter, wherein thefilter filters the multiplication result using the internal state storedin the first storage, the first storage switches an area in or fromwhich the filter writes or reads the internal state in accordance withan index signal representing a type of amplitude of a time-divisionalsignal in the measurement signal, and the second storage switches anarea in which the filtering result is stored in accordance with theindex signal.
 2. The signal detecting device according to claim 1,further comprising: a first converter that converts the measurementsignal into a digital signal; a second converter that converts thereference signal into a digital signal; a detector that detects a timingat which the index signal changes; and a re-sampler that re-samples themultiplication result at the detected timing.
 3. The signal detectingdevice according to claim 2, further comprising: a measurement signalinput terminal to which the measurement signal is input; a referencesignal input terminal to which the reference signal is input; and anindex signal input terminal to which the index signal is input.
 4. Thesignal detecting device according to claim 1, further comprising: afirst generator that generates: the reference signal based on a starttrigger signal defining a start timing of the time-divisional signal,and a timing signal representing a division timing of thetime-divisional signal; and a second generator that generates the indexsignal based on the timing signal.
 5. The signal detecting deviceaccording to claim 4, wherein the first generator generates thereference signal as a digital signal, and the signal detecting devicefurther comprises: a first converter that converts the measurementsignal into a digital signal; and a re-sampler that re-samples themultiplication result at the division timing in the timing signal. 6.The signal detecting device according to claim 5, further comprising: ameasurement signal input terminal to which the measurement signal isinput; and a start trigger signal input terminal to which the starttrigger signal is input.
 7. The signal detecting device according toclaim 1, wherein the filter performs an infinite impulse response lowpass filter in the filtering.
 8. The signal detecting device accordingto claim 1, wherein the first storage sets a new area for storing theinternal state of the filter when a value of the index signal is a newvalue.
 9. The signal detecting device according to claim 1, wherein thesecond storage sets a new area for storing the filtering result when avalue of the index signal is a new value.
 10. The signal detectingdevice according to claim 4, wherein after the start trigger signal isinput, the first generator generates the timing signal every time aperiod, with which the amplitude of the time-divisional signal changes,elapses.
 11. An optical fiber characteristics measuring devicecomprising: a first optical splitter that splits modulation light, forwhich frequency modulation has been performed, into pump light andreference light; a second optical splitter that causes the pump light tobe incident from one end of an optical fiber under test and outputsBrillouin scattering light generated inside the optical fiber; a firstdetector that detects interference light between the reference light andthe Brillouin scattering light output from the second optical splitter;an analyzer that acquires a Brillouin gain spectrum that is a spectrumof the Brillouin scattering light from a detection signal output fromthe first detector; a second detector that detects a second harmoniccomponent having a frequency that is twice a modulation frequency of themodulation light in the acquired Brillouin gain spectrum; and a measurerthat measures characteristics of the optical fiber based on the acquiredBrillouin gain spectrum and the detected second harmonic component,wherein the second detector comprises the signal detecting deviceaccording to claim 1 to which the acquired Brillouin gain spectrum isinput as the measurement signal and that detects the second harmoniccomponent using the reference signal having the frequency that is twicethe modulation frequency of the modulation light.
 12. The optical fibercharacteristics measuring device according to claim 11, wherein thesignal detecting device comprises: a first converter that converts themeasurement signal into a digital signal; a second converter thatconverts the reference signal into a digital signal; a detector thatdetects a timing at which the index signal changes; and a re-samplerthat re-samples the multiplication result at the detected timing. 13.The optical fiber characteristics measuring device according to claim12, wherein the signal detecting device further comprises: a measurementsignal input terminal to which the measurement signal is input; areference signal input terminal to which the reference signal is input;and an index signal input terminal to which the index signal is input.14. The optical fiber characteristics measuring device according toclaim 11, wherein the signal detecting device comprises: a firstgenerator that generates: the reference signal based on a start triggersignal defining a start timing of the time-divisional signal, and atiming signal representing a division timing of the time-divisionalsignal; and a second generator that generates the index signal based onthe timing signal.
 15. The optical fiber characteristics measuringdevice according to claim 14, wherein the first generator generates thereference signal as a digital signal, and the signal detecting devicefurther comprises: a first converter that converts the measurementsignal into a digital signal; and a re-sampler that re-samples themultiplication result at the division timing in the timing signal. 16.The optical fiber characteristics measuring device according to claim15, wherein the signal detecting device further comprises: a measurementsignal input terminal to which the measurement signal is input; and astart trigger signal input terminal to which the start trigger signal isinput.
 17. The optical fiber characteristics measuring device accordingto claim 11, wherein the filter performs an infinite impulse responselow pass filter in the filtering.
 18. The optical fiber characteristicsmeasuring device according to claim 11, wherein the first storage sets anew area for storing the internal state of the filter when a value ofthe index signal is a new value.
 19. The optical fiber characteristicsmeasuring device according to claim 11, wherein the second storage setsa new area for storing the filtering result when a value of the indexsignal is a new value.
 20. The optical fiber characteristics measuringdevice according to claim 14, wherein after the start trigger signal isinput, the first generator generates the timing signal every time aperiod, with which the amplitude of the time-divisional signal changes,elapses.